Electromechanical motors are increasingly being used in many different applications. An electromechanical motor creates motion and forces by intermittent friction contact between the body to be moved and has a motor stator that contains electromechanical material. Small geometrical dimensions, relatively high energy efficiency, relatively high speed and positioning accuracy are appreciated properties in most applications. However, optimizing one of these properties often results in a degradation in some other respect. For linear motion, vibration types of motors are most common, and these are based on a dynamic relative motion between driving elements and the body to be moved.
Vibration types of motors are generally driven at high frequencies. Since only a small part of the energy put into the driving elements is transformed into mechanical energy, a large portion of the energy is not used in a single cycle. In a basic design, this energy is simply dissipated as heat in the electronics or motor, which could cause severe temperature problems. The energy use thus has to be more efficient.
In order to achieve acceptable energy efficiency, it is known to use different kinds of resonance phenomena. The most common is to use a mechanical resonance of the driving elements and/or body to be moved. These prior art motors use a mechanical resonance to store the input energy as mechanical vibrations until this energy eventually is used for mechanical work. Since less electrical energy is transported forth and back to the active elements, there will be less electrical losses in this case. Furthermore, the resonance behavior makes it possible to drive the motors with lower input voltage for a given stroke. There are a huge number of resonant vibration motors. A few typical examples are going to be mentioned somewhat more in detail.
The U.S. Pat. No. 6,373,170 discloses a motor having a driving part with two separate blocks, inclined with a given angle relative to each other. The blocks operate with a so-called 33-actuation (actuating strain parallel to the electrical field) and use an interlinking drive pad to drive a rail. The entire V-shaped unit extends perpendicular to the moving rail with the individual actuators at a certain angle relative to the main displacement direction. The two actuators are driven in mechanical resonance using longitudinal vibration modes with a phase shift between the two actuators giving an elliptical trajectory of the drive pad. The phase shift between the electrodes is e.g. used to control the direction of motion.
The U.S. Pat. No. 5,453,653 discloses driving with one actuator plate with several electrodes. The actuator operates with a so-called 31-actuation (actuating strain perpendicular to the electric field) with two different resonance modes at the same time, creating an elliptical trajectory of the drive pad attached to a certain position of the actuator. One resonance mode is a bending mode and the other resonance is a longitudinal resonance mode. The longitudinal resonance mode is used for creating a motion perpendicular to the body to be moved. A phase shift between the waveforms supplied to the electrodes is used to control the direction of motion. The plate is oriented perpendicular to the moving rail.
The U.S. Pat. No. 6,392,328 discloses an arrangement with one actuator beam with several electrodes and operating with a 31-actuation at two different resonant modes. Also here, one resonance mode is a bending mode and the other resonance is a longitudinal resonance mode. This creates elliptical trajectories of the two drive pads in contact with the rail. The motion along the rail is here caused by the longitudinal resonance mode. The beam is attached to the support in the central part of the beam. By changing the phase shift of the waveforms supplied to the two drive electrodes, the direction of motion can be controlled. The beam is oriented in parallel with the rail.
However, a large drawback for mechanical resonance motors is that the actual resonance frequency is typically very sensitive even to small details of the design. Manufacturing of device parts has to be very accurate in order to achieve a predetermined resonance frequency. Also thermal and mechanical effects, such as heat expansion or wear, may alter the resonance frequency considerably. This puts demands on the electronics to be designed for compensating frequency variations. Such solutions are neither inexpensive nor small in size.
Moreover, positioning accuracy of mechanical resonance motors is often difficult to control. Also after terminating the energy input to the resonating parts, these parts will continue to vibrate until the stored energy has been dissipated in one way or another. The damping of the vibration is determined by the mechanical design factors. In general, the higher amplitude amplification that is used for the motion, i.e. the higher Q-value of the resonance, the more difficult becomes the halting control.
Another large drawback with many prior art mechanical resonance motors is that they utilize resonances of longitudinal vibrations, i.e. an extension or contraction of a piezoelectric element. The longitudinal vibrations have some disadvantages that make such solutions less attractive for small ultrasonic motors. The most severe is that the lowest longitudinal resonance frequency for a small motor will be very high. Typically a motor, one centimeter long, would have resonance frequencies above 200 kHz. This creates a problem from an electromagnetic compatibility (EMC) point of view.
In applications, where the longitudinal vibration is used for creating the actual tangential motion of the body to be moved, the driving elements have to operate with at least two contact points towards the body to be moved. Such arrangements will be larger than an arrangement with a single contact point, providing equal strokes.
The U.S. Pat. No. 6,437,485 discloses an arrangement having one actuator beam with several electrodes, operating with 31-actuation close to one fundamental resonance frequency. With a bending section in each half of the beam, activating either one or the other bending section can reverse the motion. Asymmetric driving and a frequency slightly off-resonance are used to get a 2D trajectory of the single drive pad that is placed in the center of the beam. The beam is oriented in parallel with the rail.
This type of vibrator has many advantages over other vibrators of prior art. The simple support in combination with an orientation in parallel with the rail makes it possible to build a motor with very small lateral dimensions. Furthermore, since there is no need to operate the beam in exact mechanical resonance, the drive electronics can be designed more simple. The use of only one drive pad makes it possible to get a longer stroke without making the whole motor unit larger, since the rail length can be as short as the stroke plus the width of the contact pad. The main disadvantage of this motor construction is that it is difficult to get a high efficiency and that it has to be designed carefully to function as desired.
Another resonance that can be used to improve energy efficiency is an electrical resonance. By using the actuator capacitance as a part of an electrical resonance circuit and tuning the electrical resonance to the frequency used in moving the driving elements, an improved efficiency can be reached. An electric resonance circuit is then used to store the input energy as electric or electromagnetic energy until this energy eventually is used for mechanical work. The resonance circuit will also give rise to a voltage enhancement, which allows for using lower voltage power supply.
The normal solution to create electrical resonance is to combine an inductive and capacitive component.
Several inventors have explored inductors in series or in parallel with a piezoelectric actuator. Typically the inductive component is used to reduce the resistive losses as well as transferring the stored energy to the battery or the actuator. A few inventions have concerned electrical resonance where an inductive component is connected with an electromechanical capacitive load. A typical example of a vibrator using an electrical resonance circuit is disclosed in the translated Japanese patent abstract JP 61-139284. Here an inductive element is connected in series or in parallel with a piezoelectric vibrator, which has a capacitive behavior. A commercial power source of 50 or 60 Hz is connected to the resonance circuit for providing the input power. The resonance circuit makes it possible to increase the voltage to the actuator relative the voltage of the power source.
In U.S. Pat. No. 6,459,190 an electric resonance circuit is disclosed. One inductor is connected in parallel to the piezoelectric capacitance, in order to enhance the efficiency. An additional inductor connected in series with a capacitance transforms the applied square wave to a sinusoidal wave by series resonance improving the efficiency further. The two circuits have preferably the same resonance frequency.